Transparent blends of polycarbonates and polyester have been known for several decades. Property wise they represent an amalgamation of various properties of the two polymer systems—some properties of one being boosted, but usually at the expense of the other polymer's property. Some of the areas that could use overall improvement are impact, particularly low temperature impact, solvent resistance, and high melt flow. Through the addition of a copolycarbonate system, we have maintained light transmission characteristics of the polycarbonate polyester system while significantly improving its ductility particularly at low temperature, after aging, and in the presence of steam, while having improved solvent resistance to basic organic chemical system.
One application where thermoplastic polycarbonate-polyesters blends are especially useful is in cellular telephones and other personal electronic devices. Due to continuing innovation in function and design, more robust materials are required, but such materials must also meet stringent manufacturing process requirements. For example, current design trends for cellular telephones and other personal electronic devices phones require use of in-mold-decoration (IMD) processes, in-mold-labeling (IML) processes, over-molding (or two-shot molding) processes, and thin-wall molding. Part thickness for these devices has evolved from about 1.5 to 2.0 mm down to 0.8 to 1.2 mm, and even as thin as 0.5 mm. In addition, complicated design structures are required, including lens covers (windows) with curvatures, lens covers with camera holes, integrated lens covers and housings, and the like. Lens covers must be able to provide protection to the devices inside the phone and/or allow see-through.
In IMD (also called “ink transfer” processes), since the carrier for the pattern and the ink pattern itself cannot withstand very high temperatures, materials moldable at lower injection molding temperatures are preferred, to prevent ink washout. The thermoplastic materials also advantageously have high flow, to minimize mold-in stress that can damage the carrier and the printed layer. Lower processing temperatures are also preferred for in-mold labeling (IML) and two-shot molding processes. In two-shot molding processes, it is critical that the thermoplastic material of the second shot has a lower melt temperature than the thermoplastic material of the first shot, to protect the first shot from washout or warpage caused by the hot melt of the second shot. Thin parts not only require high flow but also high impact from the material used. Industrial designers are increasing integrating several of the above design trends into one application, for example a cellular phone cover housing with a camera hole area, a transparent lens area and some geometric features, which is produced by a two-shot molding process, where the first shot is a high-temperature opaque-colored polycarbonate, and the second shot is a low temperature high flow transparent material, which is covered with IMD print and has a wall thickness of about 0.5 mm.
Acrylic resins such as poly(methyl methacrylate) (PMMA) have been used in the foregoing processes, because such resins are transparent and have a process temperature of about 220 to 230° C. However, the brittleness of PMMA and other acrylic-based resins limit their use as cellular phone lenses with curvature designs and complicated structural features. Polycarbonate alone provides high impact strength, but is processed at higher temperatures, usually about 290 to 310° C. Higher flow polycarbonates are available, but show insufficient impact strength for these applications due to the fact that the higher flow is achieved at least in part by using a lower molecular weight polycarbonate. Efforts to increase the flow of polycarbonate-polyester compositions, for example by reducing the molecular weight of the polycarbonate resin, often results in the loss of ductility. There accordingly remains a need in the art for polycarbonate-polyester blends that have high flow at lower temperatures, that can be manufactured to be transparent, and that can maintain good impact properties.